Reverse condition in Vec::retain_mut doctest

Author: ajtribick

library/alloc/src/vec/mod.rs

@@ -1470,11 +1470,11 @@ impl<T, A: Allocator> Vec<T, A> {
     ///
     /// ```
     /// let mut vec = vec![1, 2, 3, 4];
-    /// vec.retain_mut(|x| if *x > 3 {
-    ///     false
-    /// } else {
+    /// vec.retain_mut(|x| if *x <= 3 {
     ///     *x += 1;
     ///     true
+    /// } else {
+    ///     false
     /// });
     /// assert_eq!(vec, [2, 3, 4]);
     /// ```